1. Field of the Invention
This invention relates to an electrophotographic apparatus such as a copying machine and a printer, and a charging apparatus used therein. More particularly, it relates to a contact charging apparatus, and an electrophotographic apparatus, in which a charging member is brought into contact with a photosensitive member to electrostatically charge the photosensitive member.
2. Related Background Art
In charging apparatus used in electrophotography, corona charging assemblies have been conventionally used. In recent years, in place of them, contact charging assemblies are being put into practical use. The latter is intended for decreasing ozone and decreasing power consumption. In particular, roller charging systems employing a conductive roller as a charging member are preferably used in view of the stability in charging.
In the conventional contact charging, the charging is effected by the release of charges (discharging) from a charging member to an object member, and hence the charging takes place upon application of a voltage having a magnitude greater than a certain threshold voltage. For example, in an instance where a charging roller is brought into pressure contact with an OPC photosensitive member (a photosensitive member making use of an organic photoconductive material) of 25 .mu.m in layer thickness, the surface potential of the photosensitive member begins to increase upon application of a voltage of about 640 V or higher, and thereafter the surface potential of the photosensitive member linearly increases by gradient 1 with respect to the applied voltage. Hereinafter, this threshold voltage is defined as charge starting voltage Vth.
More specifically, in order to obtain a required surface potential Vd of the photosensitive member, it is necessary to apply to the charging roller a DC voltage of Vd+Vth. The method in which only a DC. voltage is applied to a contact charging member to electrostatically charge the photosensitive member by discharging is called DC charging.
In the DC charging, however, it has been difficult to keep the surface potential of the photosensitive member at the desired value because the resistance value of the contact charging member may vary depending on environmental variations and also because the Vth may vary with changes in layer thickness due to the surface scrape of the photosensitive member with its use.
Accordingly, as a proposal to achieve more uniform charging, Japanese Patent Application Laid-Open No. 63-149669 discloses an AC charging system in which a voltage formed by superposing on a DC voltage corresponding to the desired Vd an AC voltage having a peak-to-peak voltage of 2.times.Vth or higher is applied to the contact charging member. This system aims at an effect of leveling the potential by AC voltage, where the potential of the photosensitive member is converged into the Vd that is the center of the peak of the AC. voltage and can be hardly affected by external factors such as environment.
However, even in such a contact charging apparatus, its essential charging mechanism utilizes the phenomenon of discharging from the charging member to the photosensitive member. Hence, as previously stated the voltage required for the charging is required to have a value greater than the surface potential of the photosensitive member and ozone is also generated in a very small quantity. Also, when the AC charging is effected in order to achieve the uniform charging, the ozone may increase more in quantity, the electric field of the AC voltage causes vibration or noise of the charging member and photosensitive member, or the surface of the photosensitive member may seriously deteriorate due to discharging, bringing about additional problems.
Under such circumstances, as a more effective charging method, Japanese Patent Application Laid-Open No. 6-003921 discloses a method in which a charge injection layer is provided on the surface of a photosensitive member and charges are directly injected into that layer by means of a contact charging member (which is called injection charging).
In the injection charging, the charging member can be brought into contact with the photosensitive member at a greater nip between them, and it is effective to use as the charging member a magnetic brush roller which can be brought into uniform contact with the surface of the photosensitive member and can be free from microscopic incomplete charging. This is to use a charging member having the form of a magnetic brush formed by magnetically confining, using a magnet roll, ferrite particles or charging magnetic particles obtained by dispersing magnetic fine particles in a resin.
The charge injection layer serving as a surface layer of the photosensitive member may be a layer formed by dispersing conductive fine particles in an insulating and light-transmitting binder. Such a layer is preferably used. The charging magnetic brush to which a voltage is applied comes in touch with this charge injection layer, whereupon the conductive fine particles come to exist as if they are numberless independent floating electrodes with respect to the conductive support of the photosensitive member, and an be expected to have such an action that they charge he capacitor formed by these floating electrodes.
Thus, the DC voltage applied to the contact charging member without utilizing any discharge phenomenon and the surface potential of the photosensitive member are converged into values substantially equal to each other, so that a low-voltage charging method can be accomplished.
However, as to magnetic particles comprised of only iron powder, ferrite or magnetite which are conventionally used as charging magnetic particles, it is very difficult to uniformly produce those having small particle diameters.
Meanwhile, magnetic particles obtained by dispersing magnetic fine particles in a binder resin can also be used as the charging magnetic particles. However, they have tended to be broken during running if a thermoplastic resin is used as the binder resin, and the fragments of broken particles may become buried in the photosensitive member surface to tend to block exposure or affect charging performance. Accordingly, it has been attempted to use a thermosetting resin as the binder resin. Since, however, magnetic particles produced by a conventional kneading and pulverization process can not be made sufficiently spherical, such particles can not be uniformly charged and may scratch the surface of the photosensitive member in some cases. In particular, in the case of charging, different from development, there is little toner present between the magnetic particles and the photosensitive member, and hence the problem of scratch and scrape of the photosensitive member may remarkably occur.
In the injection charging, the charging member must come well into contact with the photosensitive member before the charges can be injected. However, for the magnetic resin particles produced by pulverization, it has been difficult to come well into contact with the surface of the photosensitive member, tending to result in an insufficient charging uniformity.
In addition, if magnetic particles with a broad particle size distribution are used as the charging magnetic particles, uniform charging may become impossible to cause fog on images, especially when the process speed is high or when the photosensitive member has a high surface resistance.